1. Field of the Invention
This invention relates to digital video signal processing and in particular with processing an input digital video signal representing a series of input frames to produce an output digital video signal representing a series of output frames. The invention is more particularly concerned with the case where at least some of the input frames each have a first picture portion acquired with a first acquisition characteristic (such as, for example, 60 fields/second, 2:1 interlace) and a second picture portion acquired with a second acquisition characteristic (such as, for example, 30 frames/second, 1:1 progressive scan). This invention is more particularly, but not exclusively, concerned with the case where the output signal simulates frames acquired with a different acquisition characteristic (such as, for example, 24 frames/second, 1:1 progressive scan), but is also applicable in the case where the output signal is to simulate frames wholly acquired with the acquisition characteristic of one of the picture portions of the input signal (such as, for example, 60 fields/second, 2:1 interlace).
2. Description of the Prior Art
Patent Application GB 2231228 A describes a method of converting a digital video signal in 60 fields/second, 2:1 interlace, format so as to simulate acquisition at 24 frames/second, 1:1 progressive scan format, In the simplest form of that method, for a series of ten input fields, progressive scan format frames are produced temporally aligned with the first, third, fourth, sixth, eighth and ninth of the ten input fields each either by inter-field interpolation of the respective input field with the preceding and succeeding fields, and/or by intra-field interpolation of the respective input field, depending upon the amount of motion detected in the input picture. Of a series of four corresponding output frames, the first and Fourth progressive scan converted frames can be directly used as the first and third output frames. On the other hand, the second and Fourth output frames are produced by motion compensated temporal interpolation half-way between the second and third progressive scan converted frames, and half-way between the fifth and sixth progressive scan converted frames, respectively. Motion compensated temporal interpolation involves producing, for each pixel in the output frame, a motion vector indicative of the motion of the object which that pixel represents between ,the respective pair of progressive scan converted frames. In the case of half-way interpolation, the value of each pixel in the output frame is then determined by averaging the values of the pixels located at positions in the respective pair of progressive scan converted frames offset by one-half of the motion vector one way and the other, respectively, from the position of the pixel in the output frame.
Patent Application GB 2249907 A describes a number of developments to the method described in GB 2231228 A, for example for converting 30 frames/second, 1:1 progressive scan format material by motion compensated temporal interpolation so as to simulate acquisition at 24 frames/second, 1:1 progressive scan format.
A prerequisite for satisfactory conversion using the method of GB 2231228 A and the various methods of GB 2249907 A is that the input signal has a single (actual or simulated) temporal acquisition characteristic. However, it is perceived that there is a need for this general type of conversion method which produces satisfactory results in the case where the input signal is a composite having two different temporal acquisition characteristics.